Situations arise in which it would be convenient to have a distributed means of providing heat in circumstances where heating appliances are not available. For example, producers of prepared foods have indicated that there could be significant market potential for self-heating food packaging (SHFP) systems that could heat prepared foods in their containers to serving temperature, simply, safely, and efficiently.
For a mass consumer SHFP product, safety is paramount and should be inherent; preferably there should be no extreme temperatures, no fire, no smoke or fumes under anticipated use and abuse conditions. Practical considerations mandate that any system be reasonably compact and lightweight with respect to the food to be heated. Thus, the system should have a good specific energy and high efficiency. The system must also be capable of extended storage without significant loss of function or accidental activation of the heater. There should be some simple means of activating the heating component of the system, after which the required heat load should be delivered efficiently within a specified time period, perhaps just a few minutes. Operation must be very reliable with low failure rates in millions of units of production. For a single use food application, material components should be food-safe, low-cost, environmentally friendly and recyclable.
The only SHFP technology currently in the consumer market uses an onboard system for mixing separated compartments of quicklime and water, yielding an exothermic heat of solution. These products are bulky (literally doubling package size and weight), complex, unreliable, costly, and have achieved very low market penetration. There have also been reported instances of the heater solution leaking and coming into contact with food or consumers.
An exothermic reaction in which the component reactants could be premixed yet be inert until such time as the user initiates the reaction would be beneficial in terms of providing for a simpler, more compact, and low cost package design. A solid state reaction system could offer advantage over wet chemical systems since solid systems will be less prone to spill or leak.
Thermites are a class of exothermic solid-state reactions in which a metal fuel reacts with an oxide to form the more thermodynamically stable metal oxide and the elemental form of the original oxide. Thermites are formulated as a mechanical mix of the reactant powders in the desired stoichiometric ratio. The powders may be compressed into a unitary mass. These compact reactions generate substantial heat, with system temperatures that can reach several thousand degrees, often high enough to melt one or more of the reagents involved in the reaction. However, thermite reactions typically require a very high activation energy (e.g., welding thermites [Al/FeOx] are ignited with a burning magnesium ribbon). Thus, a thermite reagent composition can be formulated to be quite stable to prevent inadvertent initiation due to electrostatic shock or mechanical impact. This generally inert character is an advantage in storage and transportation.
The most widely known thermite system is the Al/FeOx system described in Table 1. Once initiated, this system reacts virtually instantaneously to generate molten iron and is in fact used for welding rail lines. The only other significant known applications of thermites are in pyrotechnics and military weapons technologies. “A Survey of Combustible Metals, Thermites, and Intermetallics for Pyrotechnic Applications,” S. H. Fischer, M. C. Grubelich, Proc. Of 32nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference (1996) and “Thermite Reactions: their utilization in the synthesis and processing of materials,” L. L. Wang, Z. A. Munir, Y. M. Maximov, Journal of Material Science 28(14), 3693-3708 (1993) provide useful surveys of various classes of solid state reactions including thermites.
TABLE 1Characteristics of FeOx/Al and SiO2/Al Thermite ReactionsAdiabaticGasHeat ofReactionproductionDensityreactionTemperature(moles of gasReaction(g cm−3)(kJ g−1)(K)State of Productsper 100 g)2Al + Fe2O3→4.1753.953135molten Al2O3 slag0.14042Fe + Al2O3(2862° C.)Fe (liq./gas)8Al + 3Fe3O4 →4.2643.673135Molten Al2O3 slag0.05499Fe + 4Al2O3(2862° C.)Fe (liq./gas)4Al + 3SiO2 →2.6682.151889solid Al2O303Si + 2Al2O3(1616° C.)Si (liq.)
Since thermite reactions are generally vigorous with intense heat, they have not yet been successfully adapted for moderate-temperature consumer applications. Therefore, it would be highly beneficial to harness the energy release from a kinetically moderated thermite reaction thus transforming a reaction with generally pyrotechnic character to a precisely controlled power source for thermal energy and to then integrate that thermal energy into a heating device for consumer applications.